The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to HVAC systems equipped with an economizer or air handling unit utilizing outdoor air.
A typical economizer or air handling unit includes one or more dampers to control the flow of outdoor air and return air through the economizer. For efficient operation of the HVAC system, it is necessary for all of the dampers to operate properly. New regulatory requirements now necessitate that any HVAC equipment with an economizer or outdoor air damper to accurately detect when the damper(s) of an economizer or mixing box of an air handling unit become stuck or mechanically disconnected from an actuator.
In a typical application, a single actuator modulates a return air damper that is mechanically linked to an outdoor air damper. The outdoor and return air dampers are positioned in such a way that they are 180 degrees out of phase and move in unison. When the outdoor air damper is closed the return damper is fully open, and, as one damper opens the other closes. Detecting the fault conditions can be accomplished by monitoring the Supply Air Temperature (SAT) in relation to Outdoor Air Temperature (OAT) and Return Air Temperature (RAT) as the economizer modulates position, and the ratio of outdoor air to return air changes. As the economizer modulates open, the ratio of outdoor air to return air will increase, and the additional outdoor air will cause SAT to trend toward the Outdoor Air temperature. As the economizer is closed, the ratio of outdoor air to return air will decrease, and the increased return air will cause SAT to trend toward RAT. If the actuator becomes stuck or mechanically disconnected from the damper assembly, modulating the actuator will not result in a change of damper position and the ratio of outdoor air to return air will stay at the same constant ratio, and no trend in SAT will be observed. If SAT does not trend as expected when the actuator position is changed, it can be concluded that the damper is not moving as expected. The above only works, however, when the difference between OAT and RAT is large. For larger units, a single actuator may not provide enough torque to modulate both the outdoor and return dampers through a linkage assembly. In this case, it is required to attach a separate actuator to each of the outdoor and return dampers. If one actuator should become stuck or mechanically disconnected from its corresponding damper, the other actuator will continue to properly modulate its damper. With one actuator/damper pair functioning properly, the ratio of outdoor air to return air will change. The outdoor/return air ratio change will result in an SAT trend, which can lead to the false conclusion that the actuators and dampers are functioning properly.